1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, an electronic component, and a manufacturing method of a multilayer ceramic substrate.
2. Related Art
Recently, with many devices for which high performance and small size are desired such as a cellular phone, a multilayer ceramic substrate is widely in use. In a typical multilayer ceramic substrate, a plurality of ceramic substrate layers are laminated, which have conductor layer(s) internal circuit patterns are formed between the ceramic substrate layers. These circuit pattern layer(s) are connected by a conductor which is called a via conductor.
JP 2007-305740 A (hereinafter referred to as “Patent Document 1”) discloses an example structure of such a multilayer ceramic substrate. The multilayer ceramic substrate disclosed in the Patent Document 1 comprises a ceramic laminated structure in which a plurality of ceramic layers are laminated, a recess which is formed on one main surface of the ceramic laminated structure, a connection electrode which is exposed to the inside of the recess, and a terminal electrode having, as a main composition, a conductive resin which is filled in the recess and which is electrically connected to the connection electrode.
However, the multilayer ceramic substrate disclosed in the Patent Document 1 is targeted to improving a shock resistance of the connection terminal electrode with the mother substrate. Because of this, a conductive resin is required at the connection portion, and it is desired that the depth of the resin be 100 μm or greater. However, as the size of the electrode is reduced, filling of the resin becomes more difficult, and manufacturing also becomes difficult. In addition, stable attachment of metal plating on the surface of the conductive resin is not realistic. Moreover, although the Patent Document 1 discloses that a Ag—Pd alloy and a Ag—Pt alloy are suitable for high-frequency usage because these alloys have a small specific resistance, there is no consideration of the influences on the mechanical strength when the alloys are used on a surface for connecting a surface mounting component.
JP 2005-286303 A discloses that a terminal electrode on a surface is bent toward an inner layer and is covered with an insulator so that the ends are strengthened and a laminated ceramic substrate is realized which has a superior mechanical strength. However, in consideration of the positioning accuracy during the process, the size of the strengthening portion which is bent and extended toward the inner layer requires 50 μm-100 μm, and, thus, there is a limit on application to future products having a small size and high density.
JP 2001-189550 A discloses a technique in which a portion of the via hole conductor exposed to the surface is formed recessed from a surface of a multilayer ceramic substrate by 20 μm or less and a curved surface of a solder ball has good fitting to an edge of the recess so that a self positioning function (self-alignment) is achieved. However, in this Patent Document 3 also, the mechanical strength between the bump and the via hole conductor is not taken into consideration. In addition, because self-alignment is the purpose, the depth of the recess must be designed relatively deep. When the recess is formed relatively deeply as in this structure, a soldering process tends to have difficulty to reduce pores in the solder connection, resulting in a loss of mechanical or connection reliability. In this case, even if a metal plating is to be provided, because the via conductor is formed using a conductive paste having a large shrinkage ratio, the plating solution is left in the inner wall of the via hole, possibly resulting in corrosion.
On the outermost surface of the multilayer ceramic substrate, an LGA (LAND GRID ARRAY), a BGA (BALL GRID ARRAY), and a pad-shaped surface electrode having a circular or rectangular shape are formed in a complicated circuit pattern. Spacing between the electrodes is becoming narrower such as few hundreds of μm-150 μm, and, in the case of a semiconductor package component, the spacing becomes even narrower due to a flip chip mounting. Because of this, BGA which uses a solder ball of few hundred μm diameter formed on a electrode bump, on which a solder ball is mounted, is becoming to a mainstream, but in this case, the number of solder balls varies from few tens to few hundreds, and even in some cases, one thousand or more. Although the number varies depending on the usage and function of the semiconductor element, connection shear strength of about 50 gf (0.49 N) or greater is required per one electrode. This number is at a high level as a connection by a solder ball of few hundred-μm diameters.
In the case of a chip component, on the other hand, because the number of electrodes is small originally from the design requirement, the methods of using pad-shaped surface electrode having a circular or rectangular shape or the LGA are selectively used, placing higher priority on the connection strength. However, for these components also, it is desirable that the electrodes are high density and high strength.
Under the above-described circumstances, there had been a need for a multilayer ceramic substrate which has a terminal electrode structure having a high performance at high frequency, insulation reliability, corrosion resistance, and a high mechanical strength, and a manufacturing method thereof.
The present invention was conceived in view of the above circumstances, and an advantage of the present invention is that a surface layer terminal electrode is narrowed so that a size of the multilayer ceramic substrate itself is reduced and the density is increased. Another advantage of the present invention is to provide a multilayer ceramic substrate in which the connection strength of a surface mounting component or the like can be improved with a structural configuration even when the size of the terminal electrode is reduced, in which the possibility of corrosion due to the remaining of plating solution or the like is low, an electronic component which uses such a multilayer ceramic substrate, and a method of manufacturing the multilayer ceramic substrate.